add 'diff' parameter to literalinclude.

This solves issue #1134 with a pygments highlighting support.
fix encoding problem in diff param in literalinclude.

This commit creates read_with_encoding which read from file with encoding. It is used normal file read and also diff file read.
add test and document about diff params in literalinclude.
according to comment, move import, use six.text_type, and use 'not'.
use six.string_types instead of text_types.

also fix import path problem.
Merged in r_rudi/sphinx (pull request #267)

add 'diff' parameter to literalinclude.